JP-A-2006-211789 (in particular, pages 12-13, and FIG. 11) discloses a technique for preventing a battery mounted on a hybrid electric vehicle (HEV) from being overcharged and overdischarged. According to the conventional technique, an internal resistance of the battery is estimated based on a temperature of the battery, and a control range of the battery is determined based on the estimated internal resistance. The charging-discharging of the battery is controlled so that a voltage of the battery can be limited within the control range.
The battery voltage may vary due to internal conditions of the battery other than the battery internal resistance. For example, the battery voltage may vary due to a charging-discharging current of the battery and the state of charge (SOC) of the battery.
As described above, the conventional technique disclosed in JP-A-2006-211789 controls the charging-discharging of the battery based on only the internal resistance of the battery without taking into considerations other battery internal conditions such as the charging-discharging current and the state of charge of the battery. Therefore, as shown in FIG. 3, although the battery inherently has a wide charging-discharging performance range, the charging-discharging of the battery is controlled in a narrow range. As a result, the charging-discharging performance of the battery cannot be used efficiently.